(1) Field of the Invention
The present invention lies in the field of flight control systems for rotary wing aircraft, and more particularly the field of providing assistance in using flight controls, such as an autopilot.
The present invention relates to a flight control method for a rotary wing aircraft enabling it to maintain either track or heading depending on its longitudinal forward speed, and it thus forms a “full envelope” flight control method, i.e. a method that covers not only flight stages close to the ground and at low speed, but also flight stages at high altitudes and at high speed. The present invention also relates to a flight control system for a rotary wing aircraft enabling it to maintain either track or heading depending on its longitudinal forward speed.
(2) Description of Related Art
Rotary wing aircraft are aircraft that differ from other powered aircraft mainly by their ability to travel not only in cruising flight at high speeds, but also at low speeds or while hovering. This capacity is made available by using at least one main rotor of the aircraft, which rotor has an axis of rotation that is substantially vertical.
The main rotor constitutes a rotary wing providing the aircraft with lift and possibly also with propulsion. The behavior of the rotary wing aircraft in flight can be modified by varying the cyclic pitch and/or the collective pitch of the blades of the rotary wing. A variation in the cyclic pitch of the blades modifies the behavior of the aircraft in terms of attitude, and more particularly in pitching and/or in roll. A variation in the collective pitch of the blades leads to a modification in the behavior of the aircraft in terms of lift, making it possible in particular to generate movements along an axis that is substantially vertical, and also along its pitching and roll axes, depending on the attitude of the aircraft.
A rotary wing aircraft can also be maneuvered in yaw, even while stationary, by using a yaw anti-torque device. For example, such an anti-torque device is formed by a tail rotor having an axis of rotation that is substantially horizontal and located at the rear of the aircraft. Such a tail rotor has a plurality of blades, and as a general rule it is only the collective pitch of the blades that can be varied, even though it is also possible for the cyclic pitch to be variable.
A rotary wing aircraft generally has a single main rotor and a single anti-torque tail rotor. Nevertheless, a rotary wing aircraft may also have two contrarotating main rotors, e.g. in tandem or else on the same axis, in which case no anti-torque device is necessary.
Furthermore, a hybrid helicopter is a rotary wing aircraft having at least one main rotor that serves mainly to provide it with lift and to a smaller extent with propulsion, and at least one specific propulsion means such as a propulsive propeller. Such a hybrid helicopter enables large distances to be covered while traveling at a high forward speed. The anti-torque device of such a hybrid helicopter may be formed by at least one of its propulsive propellers. Such a propulsive propeller has a plurality of blades and as a general rule only their collective pitch is variable.
Furthermore, a rotary wing aircraft may have aerodynamic elements such as stabilizers, or even wings, particularly in hybrid helicopters. These aerodynamic elements may have moving parts and they can participate in making the aircraft maneuverable, in particular in cruising flight at high forward speeds.
The flight behavior of a rotary wing aircraft can be varied by modifying various flight parameters of the aircraft. These flight parameters include in particular cyclic and/or collective pitch values for the main rotors and the collective pitch value for the anti-torque rotor and/or the propulsion means, and the aerodynamic elements, if any. These flight parameters can be modified in this way in various control modes. In a manual control mode the pilot of the rotary wing aircraft has control levers that the pilot of the aircraft moves manually in order to vary the flight parameters, and in particular the cyclic and/or collective pitch of the blades of the various rotors by means of manual control linkages. The concept of “manual” should be considered in opposition to the concept of “automatic”, without prejudice to the means used by a person for maneuvering the aircraft, which means may in particular be pedals, a control stick, or a joystick.
In an embodiment of a manual control mode, control levers engage respective linkages for mechanically transmitting forces remotely, so as to enable the pilot of the rotary wing aircraft to act mechanically on the blades by using control levers, either directly, or else via servo-controls.
In another embodiment of a manual control mode, the pilot moving a control lever serves to generate electrical signals for activating at least one servo-control for moving the blades.
In an automatic control mode, an autopilot generates control signals for those flight parameters and in particular for varying the pitch of the blades of the various rotors by using automatic control linkages. When the autopilot is activated, the control signals take the place of the control signals generated by the pilot acting directly on the control levers for activating the servo-controls.
The autopilot enables the rotary wing aircraft to maintain stable progress in application of previously stored flight setpoints. The actual state of progress of the aircraft is evaluated by the autopilot at a given instant by means of information supplied by a variety of instrumentation on board the aircraft. On the autopilot detecting a difference between the flight setpoints and the actual state of progress of the aircraft, the autopilot acts on the flight behavior of the rotary wing aircraft by means of one or more flight parameters in order to re-establish its actual state of progression in compliance with the flight setpoints.
The pilot of the rotary wing aircraft activates the autopilot by using one or more specific control buttons.
In a stabilization mode performed by the autopilot, an initial setpoint for maintaining the attitude of the rotary wing aircraft may, for example, be defined relative to the state of progression of the aircraft as evaluated from activation of the autopilot. Stabilization mode serves to stabilize the aircraft by the autopilot correcting the attitude of the aircraft relative to the initial setpoint.
In a particular mode of piloting by transparency, the pilot may possibly intervene temporarily on the behavior of the aircraft by using control levers and overriding the control signals generated by the autopilot. The initial flight setpoints are unaffected, any temporary intervention by the pilot on the behavior of the aircraft not leading to any modification to the initial flight setpoints.
It is also known to correct a flight setpoint, such as a setpoint for maintaining attitude, as a function of the actual state of progression of the rotary wing aircraft after the pilot has acted on the control levers. It is also known to enable the pilot of the aircraft to correct an attitude maintaining setpoint by varying the value of that setpoint incrementally, the pilot making use of one or more dedicated control members. For example, two control members may be used of the kind commonly known as “beeps”. For example, such control members may be positioned respectively on a collective pitch control lever and on a cyclic pitch control lever generally referred to as a “stick”.
Document FR 1 347 243 describes a device for piloting by transparency that enables the pilot to take action either with a return to the initial flight setpoints after the pilot's action ceases or else with new flight setpoints being stored that take account of the action of the pilot.
Also known is Document FR 2 991 664, which describes an automatic system for providing piloting assistance that enables a flight parameter to be maintained on a progression axis of the aircraft while taking account of the action of the aircraft pilot on at least one other axis by using flight control levers while the autopilot of the aircraft is in operation. Various modes of guidance can be selected by the pilot, e.g. giving priority to maintaining a vertical speed or a forward speed or indeed maintaining heading, angle of attack, or flight path angle.
Furthermore, Document U.S. Pat. No. 5,001,646 describes an automatic control system enabling the pilot to act on the progression of the aircraft by means of a four-axis control member. The pilot can then control longitudinal, lateral, and vertical accelerations of the aircraft and also its angular speed in yaw, while conserving firstly, at low forward speed, a speed relative to the ground that is independent of the heading being followed, and secondly, at high forward speed, a coordinated turn and a flight path angle.
The rotary wing aircraft is stabilized using basic modes, in which, by way of example, the autopilot generates an increase in stability by damping angular movements of the aircraft, or indeed it serves to maintain attitude or heading. The basic modes provide piloting comfort for the pilot of the rotary wing aircraft, but they do not correct for potential differences relative to the speed or position the pilot desires for the aircraft. Proposals have thus been made to associate higher modes of operation with the basic modes in order to eliminate potential differences in position, speed, and/or acceleration of the aircraft compared with the values desired by the pilot. These desired values are input in the form of flight setpoints that the higher autopilot modes use for bringing the aircraft to the desired position, speed, and/or acceleration, and for maintaining it. The operation of stabilizing the aircraft obtained using the basic modes is performed quickly by the autopilot, whereas the operation of re-establishing position, speed, and/or acceleration of the rotary wing aircraft is performed subsequently and more slowly by the higher modes.
By way of example, Document WO 95/34029 describes a flight control system for an aircraft enabling the speeds of the aircraft to be stabilized by operating the controls relative to the yaw, roll, and pitching axes and also relative to lift, while maintaining a heading that is constant.
The autopilot can also provide advanced functions of assisting in the guidance of the rotary wing aircraft. The possibilities made available by the higher modes are also used to obtain such assistance. The ways in which advanced functions are executed depend on predefined capabilities of the autopilot relating to the setpoint track that is to be followed by the aircraft.
Specifically, such higher autopilot modes are designed to perform instrument flight rules (IFR) operations, i.e. for piloting that can be performed solely with the assistance of flight instruments and can thus be performed with degraded vision outside the aircraft, or indeed with no outside vision.
In contrast, visual flight rules (VFR) operations are performed when the pilot can control the aircraft by looking outside the aircraft and not only with the help of instruments and flight assistance.
By way of example, the setpoint track as used for a flight mission may be determined by the pilot of the rotary wing aircraft, or else during a stage of approaching a site that is known and identified. Such a site is provided in particular with means providing interactivity between the site and the autopilot, such as radio navigation beacons. In the absence of such interactive equipment, the site is identified by the pilot of the aircraft in manual mode, and then the pilot of the aircraft activates the desired advanced functions.
The operating capabilities of the autopilot make it possible to provide automatic piloting assistance by correcting the attitude of the rotary wing aircraft in cruising flight, at high forward speeds, and when the aircraft is in a position that is remote from the ground. In a stage of cruising flight, the surroundings of the aircraft are normally empty and the pilot of the aircraft does not need to pay sustained attention to the maneuvering of the aircraft. The pilot can also avoid such sustained attention close to the ground in surroundings that are known by making use of an advanced function of the autopilot, such as during a stage of approaching a landing ground that is known and/or provided with means for identifying its surroundings.
Likewise, during a stage of approaching an intervention site that is known to the autopilot and that has been recognized and identified, activation of an advanced function is made possible, even at low speeds, for guiding the rotary wing aircraft along the corresponding setpoint track.
In addition, like a person piloting an aircraft, the autopilot conventionally controls the longitudinal, lateral, and vertical speeds of the aircraft respectively by the longitudinal cyclic pitch, the lateral cyclic pitch, and the collective pitch of the main rotor, and the collective pitch of an anti-torque rotor controlling the orientation of the aircraft about its yaw axis. These longitudinal, lateral, and vertical speeds are defined in a reference frame tied to the aircraft having axes that are formed by the longitudinal, lateral, and vertical directions of the aircraft.
Furthermore, an autopilot can also enable the aircraft to perform coordinated turns. A coordinated turn is a turn performed without the aircraft drifting from the turn track relative to the ground, which is ground coordination, or else without any lateral load factor, which is air coordination.
With ground coordination, a turn is coordinated relative to the ground. The aircraft does not drift relative to the ground, thus enabling it to follow a ground track accurately. Such a turn that is coordinated relative to the ground is preferably used at low speed and low altitude so as to move safely in the proximity of terrain in relief or buildings, with the nose of the aircraft generally remaining in alignment with the ground track.
With air coordination, a turn is coordinated relative to the air. The aircraft does not drift relative to the air, thereby giving preference to the comfort of its occupants and minimizing the sideslip of the aircraft. Such a turn that is coordinated relative to the air is preferably used in cruising flight, i.e. at high speed and high altitude, and far away from any obstacles.
Document U.S. Pat. No. 5,213,283 describes a control system for performing a coordinated turn. That control system automatically supplies a yaw control signal in response to the pilot issuing a banking control signal while making such a coordinated turn, with the pilot's workload thus being reduced.
In addition, Document WO 2012/134447 describes a flight control system for an aircraft enabling a coordinated turn to be performed throughout the flight envelope, thereby minimizing the pilot's workload. At high speed, that control system makes use firstly of changes in the angle of attack of the aircraft to control heading and also lateral acceleration, and secondly of the air speed of the aircraft for controlling heading, so as to perform a coordinated turn relative to the air. At low speed, the control system makes use of the sideslip angle of the aircraft in order to maintain the heading in alignment with the track of the aircraft, thus performing a coordinated turn relative to the ground. In a transition zone between those two flight envelopes, the sideslip angle of the aircraft and its lateral acceleration are used to maintain the aircraft in a coordinated turn.
Furthermore, rotary wing aircraft are powered aircraft designed to be capable of flying in a variety of conditions that can sometimes be difficult, both in terms of atmospheric conditions, such as the presence of a strong wind and varying visibility conditions, and in terms of flight conditions, such as flying at low speeds or hovering, or indeed conditions involving the surroundings, such as being close to ground that is unknown or poorly known.
In difficult flight conditions, the pilot of the rotary wing aircraft is likely to need to take account of unexpected factors. It can then be awkward, or even impossible, for the pilot of the aircraft to make use of automatic assistance in maneuvering the aircraft under such difficult flying conditions. For example, when the aircraft is close to the ground, it must be possible for any change in its behavior to be implemented quickly. When the autopilot is using an advanced function implementing its higher modes of operation, it has difficulty in implementing a rapid modification to a track that is to be followed by the aircraft.
Under such difficult flying conditions, the use of IFR piloting can be dangerous and VFR piloting is to be preferred, but the pilot can nevertheless make use of assistance and/or certain instruments of the aircraft. Such conditions include in particular visual meteorological conditions (VMC) and degraded visual environment (DVE) conditions. The pilot may then find it necessary to make frequent adjustments to the speed and/or the track of the aircraft in order to avoid possible obstacles and in order to approach particular positions, e.g. if there is a strong side wind.
Document FR 2 777 535 describes a flight control system for an aircraft that makes it possible in particular to control lateral speed relative to the ground while maintaining a constant heading, e.g. for the purpose of compensating a strong side wind. That control system also makes it possible to maintain a constant direction for the speed of the aircraft, and thus for its track, while changing its heading and/or its longitudinal speed.
Furthermore, Document WO 2012/134460 describes a flight control system for an aircraft that makes it possible at a low speed to maintain a track that is constant relative to the ground while changing heading. The control system acts on the pitching and roll controls in order to maintain the track, with the pilot being able to cause the aircraft to move in rotation at any moment by means of those controls.
Likewise, Document WO 2012/096668 describes a flight control system for an aircraft that makes it possible to control the vertical speed of the aircraft, its flight path angle relative to the ground, and/or a height relative to the ground depending on its forward speed. Below a predetermined forward speed threshold, corresponding to a flight situation close to hovering, the flight control system makes it possible to maintain a height relative to the ground. Above that predetermined forward speed threshold, the flight control system then enables a vertical speed of the aircraft to be maintained or else it enables a flight path angle relative to the ground to be maintained.
Furthermore, Document FR 2 814 433 describes a flight control device for an aircraft in which an action on a control member can have different effects depending on the speed in translation of the aircraft. Thus, if this speed in translation of the aircraft is less than or equal to a predetermined threshold, an action on the control member acts directly on the speed in translation. In contrast, if the speed in translation of the aircraft is greater than the predetermined threshold, then an action on the control member acts, by way of example, on the acceleration in translation of the aircraft, or indeed on its angular speed.
Furthermore, Document WO 2013/012408 describes a flight control system for an aircraft that makes it possible automatically for the aircraft to engage hovering flight starting from forward flight, and also enables a position to be maintained in hovering flight.
Finally, the technological background of the field of the invention includes the following documents: WO 99/55582; EP 1 607 327; WO 93/05461; EP 2 574 546; EP 0 455 580; and WO 99/64942.